The present invention relates to the on-press processing of imaged lithographic printing plates.
Plates of interest have a solvent-soluble, radiation-polymerizable, oleophilic resin coating on a hydrophilic substrate. In conventional practice; after image-wise exposure at ultraviolet (UV), visible, or infrared (IR) wavelengths, the plates are developed with solvent to remove the unexposed areas of the coating by dissolution, thereby producing a substantially planographic pattern of oleophilic and hydrophilic areas. The developed plates are then ready for mounting on a cylinder of a printing press, where the plates are subjected to fountain fluid and ink for transfer of ink to a target surface according to the pattern of oleophilic and hydrophilic areas on the plate.
Although a process is known for developing IR imaged plates with water rather than solvent, the coating is not polymerized by the imaging. Instead, the coating contains microspheres or beads of thermally fusible material suspended in a water-soluble medium or matrix. The plate is imaged at high energy levels (250-350 mj/cm2) such that the microspheres fuse to themselves and the substrate. The imaged plates can be developed with water or fountain fluid on-press, whereby the imaged, fused areas remains intact whereas the unimaged, non-fused areas including microspheres are removed via dissolution of the matrix.
Not only is a high level of energy required for imaging such plates, but the rate of imaging is slow and the resolution is low. Also, the dissolved matrix with microspheres is a chemical waste that must be specially treated.
Thus, it should be appreciated that almost all existing negative-working planographic lithographic printing plates, with the exception of those produced by ablation in the imager, are produced by laying down a continuous film of radiation-sensitive coating on a suitable hydrophilic substrate such as grained, anodized, and hydrophilized aluminum sheet, or its equivalent, imaging the radiation-sensitive coating with actinic ultra-violet, violet, or infra-red energy in an image-wise fashion, and subsequently subtracting the non-irradiated portions of the imaged plate by the process of solubilization or dispersion, thus establishing oleophilic image areas and water receptive non-image areas.
The water soluble materials that are used in these coatings typically comprise polymers, monomers, and various components of the initiator system and in some cases additives. The purpose of using these water soluble materials is to aid in the development that takes place on press. These materials are needed to insure that the non imaged areas of the plate are completely “cleaned out” and remain water receptive. The main problem with this type of development is that these water soluble materials dissolve in the fountain solution.
Dissolution of these water soluble materials results in two very distinct problems. First, the colorant system that is used in the coating (usually a blue green material) mixes with the colored inks, causing them to shift in color. Second, the water soluble materials also act as emulsifiers which cause the ink to emulsify in the fountain solution. As a consequence, the ink and water can no longer differentiate between imaged and background areas. This causes the plate to scum in the background and image to go blind.